As integrated circuits become smaller and more dense, the metal interconnects must be made narrower, and electromigration of the metal becomes more of a problem. Aluminum has been the metal of choice for some time, because it is readily deposited, is conductive and, when mixed with silicon, has adequate electromigration properties. However, as design rules are reduced to less than one-quarter micron, the limit of line widths for aluminum is being reached.
Because of its increased conductivity, the use of copper lines and vias for small design rule integrated circuits becomes more attractive, because copper lines can be about one-half the width required for aluminum lines. However, the use of copper has been limited heretofore because elevated temperatures must be used for deposition and removal, when the danger of electromigration of the copper into adjacent devices increases.
Recently, several references have appeared directed to a chemical vapor deposition process for depositing copper films from an organic precursor, hexafluoroacetylacetonate-Cu-trimethylvinylsilane (hereinafter hfac-Cu-TMVS). This precursor dissociates at comparatively low temperatures, i.e., below 200.degree. C., to produce copper films having a low film resistivity. However, the resultant copper film has poor adhesion to underlying surfaces, particularly to TiN or Ta barrier layers, and the precursor is not particularly stable at room temperature or higher. Further, the deposition rate of copper is low.
Thus the use of various additives have been proposed for combination with the hfac-Cu-TMVS precursor to improve the film quality, deposition rate and the thermal stability of the precursor. The addition of water vapor to the copper precursor, as disclosed by Galatos et al, Appl. Phys. Lett. 63 (20), Nov. 15, 1993 pp 2842-2844, improved the deposition rate, but degraded the resistivity. Hochberg et al, in Conf. Proceed ULSI-X, 1995, of Materials Research Soc., discloses the addition of water by using the dihydrate form of hfac-Cu-TMVS. This improved the stability of the copper precursor, and the copper deposition rate. However, the copper films had low conductivity and they were rough, indicating the presence of particles. Jaim et al, in Chem. Mater. 1996, Vol. 8, pp 1119-1127, discloses the deposition of copper from hfac-Cu-TMVS and 2-butyne, also adding water to the copper precursor. However, the resistivity of the deposited copper films was high.
Nguyen, U.S. Pat. No. 5,744,192 discloses the addition of water to the above copper precursor at a chamber pressure of 0.3-3% and up to 5% by weight of additional TMVS to increase the stability of the precursor. Up to 0.4% by weight of protonated hfac (Hfac) can also be added to increase the deposition rate of the copper. However, this blend of the copper precursor is inadequate for a commercial process, and the film morphology, resistivity and purity level of the deposited copper films are inadequate.
At present, the most widely used copper precursor for CVD of copper is a blend of hfac-Cu-TMVS, an extra 2.5 percent by weight of TMVS and 0.4 percent by weight of H(fac)dihydrate (HDH). The intention of this blend is to improve the thermal stability of the copper precursor by adding extra TMVS, and to improve the deposition rate of copper by adding extra HDH. This blend is available from Schumacher Co. as "Blend 2504". However, the thermal stability of this blend is poor even with the extra 2.5% by weight of TMVS, because the HDH accelerates the decomposition of the precursor at relatively low temperatures. Decomposition of the copper precursor in the vaporizer used to pass the gaseous precursor to the deposition chamber leads to particles of deposit material being formed well before they reach the wafer, and poor adhesion of the film to the wafer. Further, because the chemical control of the blend components is difficult, the HDH concentration varies through each quantity of the blend.
The prior art method for using the above blend to deposit copper on a wafer is to add the copper precursor and blending ingredients (hfac and TMVS) to a vaporizer where the liquids are vaporized, after which the vapor passes to the CVD chamber where it decomposes and deposits copper on a substrate.
Thus a TMVS copper precursor blend that has improved stability, and an improved method of depositing copper having improved morphology, and lower copper resistivity, would be highly desirable.